This invention relates generally to radar systems, and more specifically to a radar system which is capable of synchronization with a digital elevation map (DEM) to accurately determine a location.
The proper navigation of an aircraft in all phases of its flight is based to a large extent upon the ability to determine the terrain and position over which the aircraft is passing. In this regard, instrumentation, such as radar systems, and altimeters in combination with the use of accurate electronic terrain maps, which provide the height of objects on a map, aid in the flight path of the aircraft. Electronic terrain maps are well known and are presently used to assist in the navigation of aircraft.
Pulse radar altimeters demonstrate superior altitude accuracy due to their inherent leading edge return signal tracking capability. The pulse radar altimeter transmits a pulse of radio frequency (RF) energy, and a return echo is received and tracked using a tracking system. The interval of time between signal bursts of a radar system is called the pulse repetition interval (PRI). The frequency of bursts is called the pulse repetition frequency (PRF) and is the reciprocal of PRI.
FIG. 1 shows an aircraft 2 with the Doppler effect illustrated by isodops as a result of selection by the use of Doppler filters. The area between the isodops of the Doppler configuration will be referred to as swaths. The Doppler filter, and resulting isodops are well known in this area of technology and will not be explained in any further detail. Further, the aircraft 2 in the specification will be assumed to have a vertical velocity of zero. As is known, if a vertical velocity exists, the median 8 of the Doppler effect will shift depending on the vertical velocity. If the aircraft 2 has a vertical velocity in a downward direction, the median of the Doppler would shift to the right of the figure. If the aircraft 2 has a vertical velocity in an upward direction, the Doppler would shift to the left of the figure. Again, it will be assumed in the entirety of the specification that the vertical velocity is zero for the ease of description. However, it is known that a vertical velocity almost always exists.
Radar illuminates a ground patch bounded by the antenna beam 10 from an aircraft 2. FIG. 1a shows a top view of the beam 10 along with the Doppler effect and FIG. 1b shows the transmission of the beam 10 from a side view. To scan a particular area, range gates are used to further partition the swath created by the Doppler filter. To scan a certain Doppler swath, many radar range gates operate in parallel. With the range to each partitioned area determined, a record is generated representing the contour of the terrain below the flight path. The electronic maps are used with the contour recording to determine the aircraft""s position on the electronic map. This system is extremely complex with all the components involved as well as the number of multiple range gates that are required to cover a terrain area. As a result, the computations required for this system are very extensive.
In addition to the complexity, the precision and accuracy of the distance to a particular ground area or object has never been attained using an airborne radar processor.
In one aspect, a filter is provided. The filter comprises a first order band pass filter which is configured to filter non-zero gated radar return samples and ignore a portion of received zero amplitude samples. The filter is further configured to calculate past outputs based on filter outputs generated during a previous non-zero gated radar return.
In another aspect, a method for reducing computations in a band pass filter is provided. The filter receives non-zero amplitude gated radar return samples and zero amplitude samples. The method comprises sampling the radar data with the band pass filter, processing non-zero amplitude gated radar return samples, ignoring zero amplitude samples, and calculating past outputs for the processing of non-zero amplitude, gated radar return samples based on previous filter outputs generated when processing previous non-zero amplitude, gated radar return samples.
In still another aspect, a method for processing radar return data, is provided. The method comprises sampling the radar data from each of the receiver channels, filtering the samples with a filter configured to filter non-zero gated radar return samples, configured to not process at least a portion of zero amplitude samples, and further configured to calculate past outputs based on filter outputs generated during the processing of previous non-zero gated radar return samples. Also, the filtered samples are converted to a doppler frequency, a band pass filter is applied to the doppler frequency signals, the filter centered at the doppler frequency, and a phase relationship is determined between the receiver channels.
In yet another aspect, a radar signal processing circuit is provided. The circuit comprises a radar gate correlation circuit configured sample radar data at a sampling rate, a correlation bass pass filter configured to process non-zero gated radar return samples, configured to not process at least a portion of zero amplitude samples, and further configured to calculate past outputs based on filter outputs generated during processing of at least one previous non-zero gated radar return sample. The circuit further comprises a mixer configured to down sample an in-phase component and a quadrature component of the filtered signal to a doppler frequency, and a band pass filter centered on the doppler frequency.
In another aspect, a filter comprising a digital band pass filter including an input, an output, and a sample clock input at a frequency of fsample, is provided. The filter is configured to process non-zero input samples, process a portion of zero amplitude input samples, and calculate past outputs for the processing of non-zero input samples based on filter outputs generated while processing previous non-zero input samples.